Atomic layer deposition (“ALD”) is a well known process in the semiconductor industry for forming thin films of materials on substrates such as silicon wafers. ALD is a type of vapor deposition wherein a film is built up through deposition of multiple ultra-thin layers with the thickness of the film being determined by the number of layers deposited. In an ALD process, gaseous molecules of one or more compounds (precursors) of the material to be deposited are supplied to the substrate or wafer to form a thin film of that material on the substrate. In one pulse, a first precursor material is adsorbed largely intact in a self-limiting process on the substrate. The precursor material may be decomposed in a subsequent reactant pulse to form a single molecular layer of the desired material. Alternatively, the adsorbed precursor material may react with the reactant of a subsequent reactant pulse to form a single molecular layer of a compound. Thicker films are produced through repeated growth cycles until the target thickness is achieved.
In an ALD process, one or more substrates with at least one surface to be coated are introduced into the reactor or deposition chamber. The substrate is heated to a desired temperature above the condensation temperature but below the thermal decomposition temperature of the selected vapor phase reactants. One reactant is capable of reacting with the adsorbed species of a prior reactant to form a desired product on the substrate surface. The product can be in the form of a film, liner, or layer.
During an ALD process, the reactant pulses, all of which are typically in vapor or gaseous form, are pulsed sequentially into the reactor with removal steps between reactant pulses. For example, inert gas pulses are provided between the pulses of reactants. The inert gas purges the chamber of one reactant pulse before the next reactant pulse to avoid gas phase mixing or CVD type reactions. A characteristic feature of ALD is that each reactant is delivered to the substrate until a saturated surface condition is reached. The cycles are repeated to form an atomic layer of the desired thickness. To obtain a self-limiting growth, sufficient amount of each precursor is provided to saturate the substrate. As the growth rate is self-limiting, the rate of growth is proportional to the repetition rate of the reaction sequences rather than to the flux of reactant as in CVD.
Typical reaction chambers used for ALD processing include a top plate and a bottom plate with a slot formed through the top plate. The slot allows process gases to be introduced into the reaction chamber therethrough, and the slot is a substantially linear opening arranged perpendicular to the primary access of gas flow. However, because the process gases introduced into the reaction chamber through the slot typically have the same flow velocity along the entire width of the slot, as the process gases flow through the reaction chamber, the amount of time that it takes for the process gases to contact a leading edge of the wafer differs across the width of the reaction chamber. In other words, although the velocity of process gases being introduced into the reaction chamber via the slot is substantially constant across the width of the slot, the time that it takes for the gases introduced into the reaction chamber near the edges of the reaction chamber to contact the leading edge of the substrate is greater than the time it takes for the gases introduced into the reaction chamber near the centerline of the reaction chamber to contact the leading edge of the substrate, as illustrated in FIG. 1A. Hence, the leading edge of the substrate near the centerline of the reaction chamber is exposed to a greater amount of process gases before the lateral-most edges of the substrate closest to the side walls of the reaction chamber are exposed to process gases. This typically results in the leading edge of the substrate near the centerline of the reaction chamber having a greater deposition thickness than the lateral edges of the substrate over many ALD cycles because the concentration of precursor in the process gas decreases as the precursor adsorbs to the leading edge of the substrate nearer the centerline of the reaction chamber. The decrease in precursor concentration within the process gases flowing over the substrate from the leading to the trailing edge of the substrate—and a similar decrease in concentration from the longitudinal centerline relative to the side edges of the reaction chamber—results in non-uniform deposition on the substrate. Accordingly, the ideal residence time distribution of process gases introduced into the reaction chamber through a slot should be substantially the same across the entire width of the slot such that the time that it takes the process gases to travel from the slot to a corresponding location of the leading edge of the substrate is constant across the width of the reaction chamber.
The residence time distribution (“RTD”) is a contour of constant time (i.e., the time it takes for a fluid element to reach a fixed location is constant) should be optimized such that the shape of the RTD corresponds to the entire leading edge of the substrate, as shown in FIG. 1B. Thus, there is a wave of process gases having substantially the same concentration across the entire leading edge of the substrate, from the lateral edges to the front edge of the substrate near the centerline of the reaction chamber.
Therefore, a need exists for a gas delivery system that distributes process gases such that the distributed process gases are introduced into a reaction chamber resulting in a pre-determined RTD between the slot introducing the process gases into the reaction chamber and the leading edge of the substrate to produce a more uniform film deposition across the entire substrate being processed.